The present invention relates to a device for detecting an intake air flow rate in an automotive engine, and more particularly to an improvement in such a device housed in a casing of an air cleaner or the like.
There has been priorly used a vortex-shedding flowmeter for ultrasonically detecting the generation frequency of von Karman vortices to determine the flow rate of air drawn into automotive engines. The known vortex-shedding flowmeter comprises an ultrasonic transducer or emitter for radiating ultrasonic energy into a flow of intake or suction air, and an ultrasonic transducer or receiver for receiving the ultrasonic energy as modulated by von Karman vortices generated in the air flow. The ultrasonic transducers are mounted on the wall of a pipe through which the suction air to be measured flows.
A typical prior air flow rate measuring construction, and an electronic apparatus therefor, is illustrated in U.S. Pat. No. 3,818,877 to Barrera. In this patent, the means by which the modulated ultrasonic energy is converted into a control signal for use with a fuel injector is disclosed.
It is known that the temperature of intake air in automotive engines rises 10 to 20 degrees Celsius in a few minutes when the engine is idling in the sun in midsummer. Since the propagation velocity of ultrasonic energy in air varies with temperature as is well known, the ultrasonic energy as it passes through the air to be measured is subjected to modulation due to both von Karman vortices and this temperature change while the von Karman vortices are being detected with air undergoing a sharp temperature rise. When the temperature distribution is uniform throughout the suction air, it is an easy task to detect only ultrasonic modulations due to the von Karman vortices. However, where there are temperature differences in the flow of intake air, ultrasonic modulations which are caused by the von Karman vortices and other ultrasonic modulations that are dependent upon the temperature differences are overlapped, resulting in difficulties in detecting only the vortex-dependent ultrasonic modulations. Therefore, the prior vortex-shedding flowmeter has been unable to detect von Karman vortices under such conditions.